1. Field of the Invention
The present invention relates generally to improvements to optical fiber, and more particularly to advantageous aspects of a dispersion-compensating fiber and associated module for controlling residual dispersion.
2. Description of the Prior Art
In an optical telecommunication transmission system, as an optical signal travels along an optical fiber over several kilometers, the optical signal pulse begins to spread, potentially overlapping into an adjacent pulse time slot. This phenomenon, known as wavelength dispersion, can seriously affect the integrity of the signal if not corrected before detection. More recently, the development of high speed/high bit rate systems, such as a 40 Gbit/s (DWDM), requires even greater control of dispersion compensation.
A recently developed class of optical fibers known as dispersion-compensating fiber (DCF), have steeply sloped, negative dispersion characteristics. One use for DCF is to undo the pulse spreading which occurs in fibers of positive dispersion. This is achieved by concatenating segments of fiber with positive and negative dispersion. For example, the dispersion characteristics of a DCF can be optimized to match those of already existing optical fiber links fabricated from standard single-mode fibers (SSMF) for operation over broad wavelength regions. This technique is disclosed in U.S. patent application Ser. No. 09/596,454, filed on Jun. 19, 2000, assigned to the assignee of the present application, the drawings and disclosure of which are hereby incorporated by reference in their entirety.
Conventional DCF""s typically have a dispersion versus wavelength curve, such as curve 100 shown in FIG. 1, which has a relatively large curvature. Typical transmission fibers such as standard single mode fiber (SMF), LEAF(trademark), and Truewave(trademark) fibers are more linear with positive slope. As a result, the residual dispersion curve, after compensation of the SMF with the DCF also has curvature, as shown in curve 200 of FIG. 2. This curvature is unacceptable for next-generation transmission systems. For example, for a 40 Gbit/s system operating over 800 km, the residual dispersion must be kept within a 15 ps/nm window, but as seen in FIG. 2, compensation fiber consistent with curve 200 is within 15 ps/nm for wavelengths only from about 1545 nm to 1595 nm.
The dispersion, as well as the dispersion slope, of a positive dispersion transmission fiber can be compensated over a range of wavelengths by matching the relative dispersion slope (RDS) of the DCF to that of the transmission fiber, where RDS is the ratio of dispersion slope (S) to dispersion (D):
RDS=S/D 
By using a combination of two different DCFs, very accurate control of the RDS can be achieved. However, matching the RDS of DCF to that of the transmission fiber compensates for dispersion at only one wavelength and there remains significant dispersion, referred to as residual dispersion, for wavelengths surrounding that one wavelength. The residual dispersion curve is especially problematic for ultra long distance 10 Gbit/s systems or long distance 40 Gbit/s systems. This curvature is mainly due to the curvature of the DCF dispersion curve. Thus, matching only RDS is insufficient to compensate for dispersion over a specified bandwidth.
Another type of dispersion compensation is to carefully match the RDS of DCF to that of SMF. In place of RDS, the dispersion slope compensation ratio (DSCR), which is the ratio of RDS of DCF to transmission fibers is employed:
DSCR=SDCF/DDCFxc3x97SSMF/DSMF 
However, DSCR (and therefore RDS) is not a good measure for wideband compensation, and does not guarantee an acceptable solution for dispersion compensation over a relatively wide bandwidth.
In the past, less attention was focused on compensating the dispersion slope (the variation of dispersion with wavelength), as it was viewed as a second order effect. However, with the introduction of new transmission fibers (e.g., the Corning(copyright); LEAF(trademark); and Lucent.(copyright). Truewave(trademark) optical fibers) which are optimized for wideband WDM systems, the impact of relative dispersion slope has become more significant. This is because, in general, as RDS increases, the curvature and therefore the residual dispersion also increases. With these new fibers, the relative dispersion slope can be two or more times greater than that of a conventional single mode fiber, such as the Corning(copyright) SMF-28(trademark) fiber. Therefore, as deployment of these new optical fibers progresses, there is a need for a cost-effective dispersion slope compensator to obtain accurate dispersion compensation over a relatively wide bandwidth.
The above-described issues and others are addressed by aspects of various embodiments of the present invention. One aspect of the present invention addresses a dispersion compensating module (DCM) and dispersion compensating fiber (DCF) for use in the DCM for controlling residual dispersion in a transmission system. The dispersion compensating fiber is designed with a dispersion curve having an inflection point at a wavelength near the optical transmission operating wavelengths. The dispersion curve, having an inflection point near the operating wavelength, produces a relative dispersion slope that closely matches the relative dispersion slope of the transmission fiber over a relatively wide bandwidth, around 40 nm or more, surrounding the operating wavelength range as addressed in greater detail below.
Additional features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings.